2 COMPONENTS OF THE DELAYED RECTIFIER POTASSIUM CURRENT, I-K, IN RABBIT SINOATRIAL NODE CELLS

The delayed rectifier current was studied in rabbit isolated sino-atri al (SA) node cells using the whole-cell voltage damp technique with am photericin-permeabilized patches. The envelope of tails test indicated that in SA node cells the decay of I-K(I-K,I-tail) comprises two dist inct current components similar to the specific fast and slow componen ts of I-K(I-Kr and I-Ks) that have been found in atrial and ventricula r myocytes. Dofetilide, a Class III antiarrhythmic agent and a selecti ve blocker of I-Kr, separated the delayed rectifier current into drug- sensitive current (I-Ks) and drug-insensitive current (I-Ks). The dofe tilide-sensitive current activated rapidly and it showed two component s of deactivation, the larger of which was very slow, while the dofeti lide-insensitive current activated more slowly and deactivated quickly . The effects of propofol, an I-Ks blocker, on the delayed rectifier c urrent were also investigated. The results show that I-Ks contributes to I-K,I-tail and that the more positive the membrane potential, the m ore the contribution of I-Ks. The ratio of I-Ks to I-Kr in tail curren ts al -40 mV after a 1 s clamp pulse to +40 mV was 0.3-0.4:1. Dofetili de slowed spontaneous activity, suggesting that I-Kr contributes to th e pacemaker activity of the SA node cell.